IN JULY OF 1979, Ronald Reagan, then eighteen months from the presidency, was taken to see the North American Aerospace Defense Command, better known as NORAD. The underground facility, jointly run by the United States and Canada, is carved inside Cheyenne Mountain, near Colorado Springs. In one widely cited account of the visit, many on the tour were visibly awed by the scale and seriousness of the operation. But when Reagan asked what the US could do to stop a nuclear missile, the answer shocked him: nothing.

As the story goes, Reagan was told that all NORAD could do was track incoming warheads and provide information for retaliation. During the flight home, one aide remembered, Reagan “couldn’t believe the United States had no defense against Soviet attack. He slowly shook his head and said, ‘We have spent all that money and have all that equipment, and there is nothing we can do to prevent a nuclear missile from hitting us.’”

Reagan agonized over the idea of the US being vulnerable. “We should have some way of defending ourselves,” he concluded. His vision eventually took the form of the Strategic Defense Initiative: a plan for futuristic weapons in space—lasers, interceptors, armed satellites—that would render nuclear missiles “impotent and obsolete.” SDI was a promise as sweeping as it was speculative, and it ultimately petered out under the weight of its technical limits and astronomical costs.

After Reagan left office, his successors, George H. W. Bush and Bill Clinton, quietly but significantly pared back SDI, largely shelving the space-based part and concentrating on land-based interceptor missiles that could meet a much more limited threat. About two decades later, George W. Bush went forward with this version of the idea. His system was designed to defeat not thousands or even hundreds of weapons launched by a peer adversary but to stop a handful of missiles from a so-called rogue state. Though something workable was produced, it, too, fell short of ambitions (only about half of its highly scripted test interceptions have worked).

Now Donald Trump has unveiled his own iteration of Reagan’s old aspiration: the Golden Dome. He claims it will cost $175 billion (US), be completed by the end of his term, be 100 percent successful, and thus be capable of “forever ending the missile threat to the American homeland.” The plan has notable supporters, mainly Republicans, defence hawks, and industry players. Few credible experts believe the hype. The American Enterprise Institute, a conservative think tank, estimates the cost could rise to more than $3 trillion (US) and the system could take decades to build—if it can ever succeed.